How to find a habitable exoplanet: Don't look for one

Most planetary scientists will tell you that the objects they study are more complex and harder to categorize than almost anything else out there in the universe. That assertion is surprising and interesting, and it's a point that is gradually sinking in for astronomers. Much of the past 400 years of telescopic exploration has been about stellar taxonomy, pinning objects into their respective display cases. Despite the glorious wealth of 200 billion stars in our galaxy, the physics underlying their fundamental properties is remarkably uniform, and most can be readily described with only a few parameters—mass, age, elemental abundance. Individuals may be having particularly bad millennia, covered in spots, twisting their magnetic fields into knots, and throwing off flares, but their overall place in the cosmic zoo can be well defined.

Not true for planets, especially the smaller ones that just might be suitable for harboring life in a recognizable form. Mass, age and composition are just the start of a lengthy list of important characteristics. How far does it orbit from its parent star? What type of star does it orbit? Is the orbit elliptical? Does the planet have an atmosphere, and if so what is the composition? Is there an axial tilt? Are there other planets in the system, exerting their gravitational might and forcing the orbit to shift over time? Are there gravitational tides at work, flexing and molding the planet? Is there volcanism or tectonic activity? How is the interior of a world layered? Is there a global magnetic field, cocooning the world? How did the planet assemble, does it have surface water? Did the planet get washed by rich organic chemistry in its youth? Does it have moons, and if so, what are the conditions on those? How often are they pelted by asteroids and comets?

It’s headache inducing. We can already guess that nature is far more creative than we are in building a diversity of planets. This raises a very real, practical problem. Any present or future, real or imagined, astronomical instrument is hogtied by the constraints of time. Space observatories have finite lifetimes because of consumables like coolants and propellants, not to mention budgets. Terrestrially bound devices compete against weather and wear and tear. So the question is: here we are, a civilization reaching the point at which we can seriously look for other planets that might harbor life, and perhaps remotely gather evidence for that life, but how do we choose which members of the exoplanetary zoo to spend our precious time on?

A fair amount of work has been done on trying to answer precisely this question. It typically comes down to trying to balance out the need to look for stuff that we might recognize – real analogs to the Earth – with what nature actually offers up in our stellar neighborhood. For example, 75 percent of all stars are less than half as massive as the sun. The great majority of our nearby cousins are these dim, reddish and very long-lived objects. Earth-size planets in these systems may have to orbit very close to their parent stars to maintain a temperate surface climate, and this requirement brings a number of new issues to the table. Tides could lock these worlds into having a permanent day and night side, and stellar flares could be severe at these distances, stripping off planetary atmospheres and irradiating the surfaces. The bottom line is that conditions on these planets might or might not be pleasant, and that uncertainty affects the odds of spotting signs of life.

On the face of it, this suggests we could be in for a long hard scientific slog. However, the truth is that we’re strongly driven by the desire to know whether any life exists beyond the confines of the Earth. Is there a way to forego something in exchange for an answer to that general question? I think there may be a way in the future, and it’s the likely diversity of planets that offers a clue.

Maybe, just maybe, we should resist the urge to race to the finish on a few hazardously preselected worlds and instead look at the big picture. Suppose there are biospheres scattered across many systems, perhaps driven by the same kind of microbial machinery that dominates our own. Conditions can vary tremendously among these worlds, but biospheres still persist. Environments on such planets may be held in subtly different equilibria than their sterile equivalents—seen through atmospheric composition, reflectivity, and temperature. No single world may actually present enough of a smoking gun to let us say, "there be life," but put the data from all these planets together, and that signature might be detectable. Statistics are wonderful, if finicky, things. They let us cut through the haze and see things we would otherwise never find. Suppose we accumulate crude, rudimentary data on not just a few planets, but on hundreds or thousands. No single observation of a planet may tell us if it is teeming with life, but the cumulative weight of different parts of the planetary zoo might at least tell us if there is life on some of them. By letting go of our desire to locate a single instance of life, we’d stand to answer the global question.

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